Researchers Discover Mechanism Allows Organic Solar Cells to Create a Charge

Saturday, January 13, 2018 - 10:54

Researchers at Purdue University have discovered the mechanism that allows organic solar cells to create a charge, solving a longstanding puzzle in physics.

Organic solar cells are built with soft molecules, while inorganic solar cells, often silicon-based, are built with more rigid materials. Silicon cells currently dominate the industry, but they're expensive and stiff, while organic cells have the potential to be light, flexible and cheap. The drawback is that creating an electric current in organic cells is much more difficult, Phys.org reports.

To create an electrical current, two particles, one with a negative charge (electron) and one with a positive charge (electron-hole), must separate despite being bound tightly together. These two particles, which together form an exciton, usually require a manmade interface to separate them. The interface draws the electron through an electron acceptor and leaves the hole behind. Even with the interface in place, the electron and hole are still attracted to each other – there's another mechanism that helps them separate.

"We discovered that this type of electron-hole interface is not one single static state. The electron and the hole can be far apart or close together, and the farther apart they are, the more likely they are to separate," said Libai Huang, an assistant professor of chemistry in Purdue's College of Science, who led the research. "When they're far apart, they're actually very mobile, and they can move pretty fast. We think that this kind of fast motion between the positive and negative charge is what's driving separation at these interfaces."

"It's really hard to do optical spectroscopy at that length scale. These states also don't live very long, so you need a time resolution that's very short," said Huang. "We developed this tool called ultrafast microscopy in which we combine time and spatial resolution to basically look at processes that happen at fast time scales in very small things."

Knowing how excitons separate could help researchers design new interfaces for organic solar cells. It could also mean there are materials to build solar cells with that have yet to be harnessed, said Huang.

The paper published Friday (Jan. 12) in the journal Science Advances.

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